The present invention relates generally to semiconductor device manufacturing and process control, and specifically to monitoring the thickness and composition of ultra-thin layers deposited on semiconductor wafers.
Auger electron spectroscopy (AES) is a well-known technique for measuring chemical and compositional properties of materials. The principles of AES and its use in various analytical applications are described, for example, by Kazmerski, in Chapter 4 of Microanalysis of Solids (Plenum Press, New York, 1994), which is incorporated herein by reference.
AES is based on measuring the energy spectra of characteristic electrons, referred to as Auger electrons, which are emitted from a solid that is irradiated by an energetic ionizing beam, typically an electron beam. The beam causes ionization of atomic core levels in the solid. The resulting core-level vacancy is immediately filled by another electron from a higher energy level in a radiationless process. The energy released by the transition of the electron from the higher level to the core level is transferred to another electron in the same level or close to it, enabling this latter electronxe2x80x94the Auger electronxe2x80x94to escape from the atom. The kinetic energy of the Auger electron is determined by the work function of the atom and the structure of its energy levels.
Because of the nature of the Auger process, every element (except hydrogen and helium) is characterized by a unique Auger spectrum, with well-defined energy peaks. These spectra are well documented in the scientific literature. By analyzing the distribution and amplitudes of the lines in the Auger spectrum of a given material, it is possible to determine the elemental components of the material and their relative concentrations. Analytical systems based on AES are commercially available. For example, the xe2x80x9cSMART-Tool,xe2x80x9d produced by Physical Electronics, Inc. (Eden Prairie, Minne.) uses AES to identify and analyze microscopic defects and contamination on semiconductor wafers and magnetic drive heads.
Auger electrons created by the incident ionizing beam must escape from the sample in order to be detected and analyzed. After the Auger electrons break free of their host atoms, however, they rapidly undergo energy losses due to collisions and other phenomena. In semiconductors and metals, the mean free paths of Auger electrons are typically on the order of 0.4 to 5 nm. Therefore, only Auger electrons created very near the surface of the material under study have a significant probability of escaping and being detected. As a result, AES provides data only on the uppermost surface layers of the material. This feature of AES is clearly advantageous in surface and thin film analysis applications.
A number of new processes in semiconductor device fabrication involve the formation of ultra-thin surface layers, with thickness in the range of 1 nm or less. For good, consistent device performance, the thickness and composition of such layers must be measured and precisely controlled. In order to maximize yield and correct process errors, the thickness and composition measurements should be made while the wafers are in process, after the ultra-thin layer has been formed, and before the next layer is deposited over it. If the wafer is exposed to ambient air before forming the next layer, however, the desired properties of the ultra-thin layer may be ruined by oxidation and/or water adsorption. The wafer cannot thereafter be returned to the production chamber. There is therefore a need for compact tools that are capable of rapidly and accurately measuring the thickness and composition of ultra-thin layers in the production environment.
In response to this need, preferred embodiments of the present invention provide a fast, inexpensive Auger metrology chamber, which can be integrated with cluster tools used in semiconductor wafer fabrication. The chamber comprises an electron gun, which irradiates a small, selected spot on the wafer surface, and an analyzer, which measures the resultant Auger electron spectrum. The spectrum includes atomic emission peaks both from the ultra-thin layer on the wafer surface and from the underlying layer below it. The relative intensities of the peaks are used to determine both the composition of the ultra-thin layer and its thickness. This information can be used to detect and correct process defects in situ, without ruining the ultra-thin layer by exposing it to ambient air outside the cluster tool.
Although preferred embodiments described herein are directed specifically to fabrication of microelectronic devices on semiconductor wafers, the principles of the present invention may similarly be applied to measuring composition and thickness of ultra-thin layers formed on substrates of other types.
There is therefore provided, in accordance with a preferred embodiment of the present invention, apparatus for analysis of a thin film formed over an underlying layer on a surface of a sample, the thin film including first elements, while the underlying layer includes second elements, the apparatus including:
an electron gun, which is adapted to direct a beam of electrons to impinge on a point on the surface of the sample at which the thin film is formed;
an electron detector, which is adapted to receive Auger electrons emitted by the first and second elements responsive to the impinging beam of electrons, and to output a signal indicative of a distribution of energies of the emitted electrons; and
a controller, which is coupled to receive the signal and to analyze the distribution of the energies so as to determine a composition of the first elements in the thin film and a thickness of the thin film.
Preferably, the controller is adapted to find first and second peaks in the distribution of the energies corresponding respectively to the Auger electrons emitted by the first and second elements, the peaks having respective amplitudes, and to analyze the amplitudes of the first second peaks in order to determine the composition and thickness of the thin film. Most preferably, the controller is adapted to compare the amplitudes of the second peaks to the amplitudes of the first peaks in order to estimate an attenuation of the Auger electrons emitted by the second elements, so as to determine thereby the thickness of the thin film. Additionally or alternatively, the controller is adapted to compare the amplitudes of the first peaks one to another so as to determine the composition of the thin film.
There is also provided, in accordance with a preferred embodiment of the present invention, a cluster tool for producing microelectronic devices, including:
a deposition station, which is adapted to form a thin film including first elements over an underlying layer on a surface of a semiconductor wafer, the underlying layer including second elements;
a testing station, including:
an electron gun, which is adapted to direct a beam of electrons to impinge on a point on the surface of wafer at which the thin film is formed;
an electron detector, which is adapted to receive Auger electrons emitted by the first and second elements responsive to the impinging beam of electrons, and to output a signal indicative of a distribution of energies of the emitted electrons; and
a controller, which is coupled to receive the signal and to analyze the distribution of the energies so as to determine a composition of the first elements in the thin film and a thickness of the thin film, and to adjust an operating parameter of the deposition station responsive to at least one of the composition and the thickness.
Preferably, the tool includes a robot, which is adapted to transfer the wafer from the deposition station to the testing station, while the wafer is maintained in a vacuum.
In a preferred embodiment, the thin film formed by the deposition station includes a gate dielectric layer. In another preferred embodiment, the deposition station is adapted to form the thin film by atomic layer deposition. Preferably, the thin film formed by atomic layer deposition includes a conductive barrier layer formed in preparation for deposition of a metal seed on the surface.
Preferably, the thickness of the thin film is less than 50 nm, more preferably less than 5 nm, and most preferably less than 1 nm.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for production testing, including:
receiving a sample including a thin film formed over an underlying layer on a surface of the sample, the thin film including first elements, while the underlying layer includes second elements;
directing an electron beam to impinge on a point on the surface of the sample at which the thin film is formed;
receiving Auger electrons emitted by the first and second elements responsive to the impinging beam of electrons; and
analyzing a distribution of energies of the emitted electrons so as to determine a composition of the first elements in the thin film and a thickness of the thin film.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: